防线 means "defensive line" or "line of defense" and refers to a physical or strategic barrier set up to resist attack. It is used in military and sports contexts alike.
敌军突破了我们的第一道防线。
díjūn tūpò le wǒmen de dì yī dào fángxiàn.
The enemy forces broke through our first line of defense.
球队的防线今天表现得非常稳固。
qiúduì de fángxiàn jīntiān biǎoxiàn de fēicháng wěngù.
The team's defensive line performed very solidly today.
他们在边境建立了一条坚固的防线。
tāmen zài biānjìng jiànlì le yī tiáo jiāngù de fángxiàn.
They established a strong line of defense along the border.